The present invention relates to a method of manufacturing a cathode for a magnetron which has an extended life and has a low cost to manufacture.
Hitherto, there is coated a powdery electron emitting material composed of an oxide of barium, strontium, calcium, or the like onto a cathode of magnetron so that sufficient electron can be emitted from the material so that sufficient electron can be emitted from the material by heating the cathode with a heater. The electron emitting material is partly evaporated due to thehigh temperature of cathode during an operation, and also is damaged and eroded because a part of the electron emitted from the electron emitting material returns to the surface of the electron emitting material as back bombardment under the influence of the applied magnetic field.
Furthermore, a magnetron is usually operated with a pulse current whereby a large peak-current flows. In this case, there occurs a local spark or arc due to low electrical conductivity of the electron emitting material and the electron emitting material is also damaged.
In order to improve the above-mentioned drawbacks, cathodes of which construction are shown in FIG. 7 and 8 are proposed so as to lower thedamage or erosion of the electron emitting material. In FIG. 7, numeral 1 is a heater composed of tungsten, or the like, and numeral 2 is a hollow cylindrical cathode sleeve composed of nickel, an alloy thereof, or the like. The heater 1 is contained in the sleeve 2. On the sleeve 2, end-shields 3 and 4 are provided at such interval that corresponds to the width of a vane (anode). The outer surface of the sleeve 2 between the end-shields 3 and 4 is coated with a mixed electron emitting material 5 obtained by mixing a metal powder with the above-mentioned electron emitting material. In this embodiment, the metal powder lowers the effective resistance of the electron emitting material and protects the electron emitting material against the electron back bombardment.
However, in the arrangement of FIG. 7, when the cathode is used for a long term, the electron emitting material in the above-mentioned mixture 5 decreases and becomes thin by erosion or evaporation, and then the surface of the above-mentioned mixture 5 becomes rich in metal. Accordingly, there is generated a problem that the efficiency of electron emission, is lowered since the secondary electron, emission caused by a back bombardment of the emitted electron is reduced.
In the arrangement of FIG. 8, the outer surface of the sleeve 2 between the end-shields 3 and 4 is covered with a metal mesh 6 which is welded onto the outer surface of the sleeve, and an electron emitting material 7 is applied onto the surface so that the meshes of the metal mesh 6 are filled with the electron emitting material 7. In this embodiment, the metal mesh 6 functions as the above-mentioned metal powder.
In the arrangement of FIG. 8, however, the metal mesh 6 is easily peeled off from the surface of the sleeve 2 due to thermal-stress caused by the repetition of on-off action of a magnetron. In such a case, the electric resistance is increased and the electron emitting material 7 is badly damaged, and there are caused various troubles due to the increase of thermal resisance.
Further, in order to improve the above drawback due to the peeling off of the metal mesh from the cathode sleeve and thereby to improve the reliability of a magnetron, there is proposed a magnetron wherein metal walls defining recesses are formed integrally with a cathode sleeve as in disclosed in U.S. Pat. No. 4,380,717. However, the principal consideration of the above prior art is to make the metal walls defining recesses substantially parallel (or to make the recesses to be undercut portions). Also, there is a large labor cost to manufacture a cathode sleeve having the above mentioned shape, whereby raising the cost of manufacturing a cathode, i.e. a magnetron. Further, since the metal walls are made substantially parallel, electron emissive material is hard to deposite downward due to its shrinkage or sinter caused by the rise in temperature of a cathode, whereby voids are apt to generate within electron emissive material. In result, there are caused the same problems as in the example shown in FIG. 8 that electric resistance increases and the temperature of electron emissive material unusually rises.
It is an object of the present invention to provide a method of manufacturing a cathode for magnetron wherein such a problem as generation of voids within electron emissive material do not take place ever if the cathode is elevated in temperature; manufacturing processes are easy; and the manufacturing cost is low.
This and other objects of the invention will become apparent from the description hereinafter.